USGS Washington Water Science Center
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9722-BAK - Nitrate trends in ground water of the Columbia Basin Ground Water Management Area - Completed FY2004
Problem - The Columbia Basin Ground Water Management Area (GWMA) includes Adams, Franklin, and Grant Counties in eastern Washington, encompassing an area of 5,985 mi2. The Columbia Basin is one of the Nation's top two producers of potatoes and wheat and is a significant producer of apples and many specialty crops. Much of the southeastern part of the GWMA is intensively irrigated with Columbia River water, while dryland or ground-water-irrigated farming and rangeland grazing dominate other parts of the GWMA. Over eighty percent of drinking water in the Columbia Basin comes from ground water.
In a recent study of nitrate concentrations in ground water in Adams, Franklin, and Grant Counties, 23 percent of the more than 500 wells that were sampled had nitrate concentrations that exceeded the U.S. Environmental Protection Agency maximum contaminant level for nitrate (10 milligrams per liter) and an additional 37 percent had concentrations between 3 and 10 milligrams per liter (Ryker and Frans, 2000). Concentrations greater than 3 milligrams per liter are usually indicative of anthropogenic impacts on the ground water (Madison and Brunett, 1985). The Columbia Basin GWMA was formed in 1998 with the goal of reducing the concentrations of nitrate in ground water of the area. Agricultural best management practices are being implemented in an attempt to reach this goal.
Over 500 wells were sampled for nitrate in the fall of 1998, 2000, and 2002. Additionally, a subset of approximately 50 wells in Franklin County was sampled for nitrate in 1986, 1991, and 1995. The wells are distributed over 17 sampling regions and can be roughly divided into shallow and deep depth categories (Ryker and Frans, 2000).
Objective - The objective of this study is to determine if there are trends in the nitrate concentrations in ground water in the GWMA from 1998 through 2002 using a dataset of over 500 wells and to determine the trend in a subset of approximately 50 wells from 1986 to 2002.
Relevance and Benefits - This study will utilize one of the largest ground-water nitrate trend datasets in the United States to address the second major mission goal of the USGS 2000-2005 Strategic Plan of better understanding the Nation's environment and natural resources. The study will help determine whether the implementation of agricultural best management practices has resulted in a downward trend in nitrate concentrations in ground water. The study also addresses one of the water issues identified in the USGS Washington Water Science Center Science Plan - water quality and, specifically, basin-wide assessments of contaminants in ground water. Locally, the study results are important to the Columbia Basin GWMA, Washington State Department of Ecology, and other parties who desire to resolve questions about the trends of nitrate concentrations in the ground water of the Columbia Basin. This study is a continuation of previous work between the USGS and the GWMA.
Approach - A matched-pair test will be used to determine if there is a statistically significant increase or decrease in nitrate concentrations in the dataset of over 500 wells as a whole as well as by sampling region. The type of matched-pair test to be used will be determined after checking the dataset for symmetry and normality. The matched-pair test will establish whether the nitrate concentrations from a given sampling year are larger or smaller than those of a different sampling year with a certain confidence level. An overall Kendall's test for trend will determine the typical magnitude of trend across the entire area. Additionally, for the subset of approximately 50 wells sampled since 1986, a Kendall's tau will be computed to obtain a trend of the nitrate concentrations for each well. Kendall's tau provides a measure of the strength of the relationship between the nitrate concentrations and time (Helsel and Hirsch, 1992).
WA420 - Columbia Basin Ground Water Management Area - Completed FY2001
Background & Problem - Over eighty percent of drinking water in the Columbia Basin, eastern Washington, comes from ground water. In Adams, Franklin, and Grant Counties, approximately 20% of all sampled wells have nitrate concentrations exceeding the U.S. Environmental Protection Agency maximum contaminant level for nitrate (10 milligrams per liter). In February 1998, the Washington State Department of Ecology approved the formation of the Columbia Basin Ground Water Management Area (GWMA), with an initial emphasis on the reduction of nitrate concentrations in ground water. The three counties have agreed that water characterization and monitoring should be carried out by a partnership of the USGS and Kennedy/Jenks Consultants.
Characterization & Monitoring Tasks - The USGS role includes the design and analysis of regional-scale nitrate sampling to determine the horizontal and vertical distribution of nitrate; analysis of environmental and anthropogenic factors' influence over nitrate concentration; and design of a regional-scale long-term nitrate monitoring plan. Kennedy/Jenks' role includes coordinating sampling, analysis of site-specific nitrate trends, and design of long-term monitoring plan to assess the effectiveness of Best Management Practices. All sampling is being conducted by local Conservation Districts and Health Districts. The USGS project manager is Lonna Frans (firstname.lastname@example.org, 253/428-3600 x2694); the Kennedy/Jenks project manager is Said Amali (SaidAmali@KennedyJenks.com, 253/942-3408).
Objectives and Approach
(1) Determine the horizontal and vertical distribution of nitrate concentrations in ground water of Adams, Franklin, and Grant Counties in eastern Washington.
Approach: In FY 1998, statistically summarize existing nitrate data for shallow and deep wells, and identify gaps in existing data sets. Develop a mass sampling plan for fall 1998, to fill gaps in existing data sets. In FY 1999, incorporate new nitrate data from the mass sampling, and analyze for temporal trends. Complete report on trends and results of the mass sampling.
(2) Perform a risk analysis quantifying the relationships of environmental and anthropogenic factors to high concentrations of nitrate.
Approach: In FY 1999, correlate existing and new nitrate data with factors such as well depth, distance from irrigation canals, surrounding land use (i.e. crops grown), and soil type. In FY 2000, complete a report on results of the correlation analysis.
(3) Develop a long-term monitoring plan to identify temporal trends in regional nitrate concentrations in ground water, and their potential impact on future availability of low-nitrate drinking water.
Approach - In FY 1999, select wells and determine the frequency of sampling needed to identify regional and local trends.